A nearby department store was holding a going-out-of-business sale, so researchers from the lab went off to fill their car trunks with simple products — not exactly wedding-quality gifts, but sufficiently appealing to interest college students. When they came to the lab, the students were told they would get to keep one item at the end of the experiment, but first they had to make a series of choices. Would they prefer a pen or a candle? A vanilla-scented candle or an almond-scented one? A candle or a T-shirt? A black T-shirt or a red T-shirt? A control group, meanwhile — let’s call them the nondeciders — spent an equally long period contemplating all these same products without having to make any choices. They were asked just to give their opinion of each product and report how often they had used such a product in the last six months.

Afterward, all the participants were given one of the classic tests of self-control: holding your hand in ice water for as long as you can. The impulse is to pull your hand out, so self-discipline is needed to keep the hand underwater. The deciders gave up much faster; they lasted 28 seconds, less than half the 67-second average of the nondeciders. Making all those choices had apparently sapped their willpower, and it wasn’t an isolated effect. It was confirmed in other experiments testing students after they went through exercises like choosing courses from the college catalog.

This is a rough and ready test of conscientiousness, but it is worth remembering that conscientiousness is a very big bucket. There are lots of sub-traits that fall in this category. Here is a list of sub-traits from one test:

Self-Efficacy

Orderliness

Dutifulness

Achievement-Striving

Self-Discipline

Cautiousness

The sub-traits have formal similarities, but they can actually have a complete lack of correlation. My sub-trait scores on C have almost exactly zero correlation.

An interesting result from the work of Baumeister: eating restores willpower.

The researchers set out to test something called the Mardi Gras theory — the notion that you could build up willpower by first indulging yourself in pleasure, the way Mardi Gras feasters do just before the rigors of Lent. In place of a Fat Tuesday breakfast, the chefs in the lab at Florida State whipped up lusciously thick milkshakes for a group of subjects who were resting in between two laboratory tasks requiring willpower. Sure enough, the delicious shakes seemed to strengthen willpower by helping people perform better than expected on the next task. So far, so good. But the experiment also included a control group of people who were fed a tasteless concoction of low-fat dairy glop. It provided them with no pleasure, yet it produced similar improvements in self-control. The Mardi Gras theory looked wrong. Besides tragically removing an excuse for romping down the streets of New Orleans, the result was embarrassing for the researchers. Matthew Gailliot, the graduate student who ran the study, stood looking down at his shoes as he told Baumeister about the fiasco.

Baumeister tried to be optimistic. Maybe the study wasn’t a failure. Something had happened, after all. Even the tasteless glop had done the job, but how? If it wasn’t the pleasure, could it be the calories? At first the idea seemed a bit daft. For decades, psychologists had been studying performance on mental tasks without worrying much about the results being affected by dairy-product consumption. They liked to envision the human mind as a computer, focusing on the way it processed information. In their eagerness to chart the human equivalent of the computer’s chips and circuits, most psychologists neglected one mundane but essential part of the machine: the power supply. The brain, like the rest of the body, derived energy from glucose, the simple sugar manufactured from all kinds of foods. To establish cause and effect, researchers at Baumeister’s lab tried refueling the brain in a series of experiments involving lemonade mixed either with sugar or with a diet sweetener. The sugary lemonade provided a burst of glucose, the effects of which could be observed right away in the lab; the sugarless variety tasted quite similar without providing the same burst of glucose. Again and again, the sugar restored willpower, but the artificial sweetener had no effect. The glucose would at least mitigate the ego depletion and sometimes completely reverse it. The restored willpower improved people’s self-control as well as the quality of their decisions: they resisted irrational bias when making choices, and when asked to make financial decisions, they were more likely to choose the better long-term strategy instead of going for a quick payoff.

Again, not too surprising for me. I've known for a long time that I lose my temper when I get hungry. People who have more C can suffer fools gladly longer than I can when hungry. Since mental energy is material, this is to be expected. There is a fun Newtonian twist to this. One of Baumeister's students didn't believe that glucose could really affect willpower. He proved that overall energy usage didn't really change in the brain, no matter how much willpower the subject had. What he didn't expect, however, was that there was an equal and opposite reaction in which areas of the brain receive energy when your willpower is depleted, and the balance is restored by eating.

The results of the experiment were announced in January, during Heatherton’s speech accepting the leadership of the Society for Personality and Social Psychology, the world’s largest group of social psychologists. In his presidential address at the annual meeting in San Antonio, Heatherton reported that administering glucose completely reversed the brain changes wrought by depletion — a finding, he said, that thoroughly surprised him. Heatherton’s results did much more than provide additional confirmation that glucose is a vital part of willpower; they helped solve the puzzle over how glucose could work without global changes in the brain’s total energy use. Apparently ego depletion causes activity to rise in some parts of the brain and to decline in others. Your brain does not stop working when glucose is low. It stops doing some things and starts doing others. It responds more strongly to immediate rewards and pays less attention to long-term prospects.

If you wish, you can apply the standard evolutionary biology mental shortcut at this point.

One of the virtues of the Aristotelian account of the virtues is that as you become more experienced in living a virtuous life, good choices become habits that no longer require thought. This frees up your mental energy for bigger and better things.